Shape memory alloys exhibit thermo-mechanical properties that are useful in constructing thermally actuatable devices. Generally, a shape memory alloy (SMA) is a metallic alloy that has distinctly different phases on opposing sides of a transition temperature. An SMA reaches a first physical state when it is below its transition temperature and a second physical state when it is above its transition temperature. The warmer second physical state is generally thought to have higher degrees of solid-phase crystalline order and symmetry than the cooler first physical state. The first and second states are typically referred to respectively as martensitic and austenitic states or phases in literature related to SMA technologies.
Though a transformation from one of these states to the other does not necessarily produce an observable macroscopic shape change, some SMA materials can be trained to have a first shape for the cooler first state and a second shape for the warmer second state. A two-way trained SMA can forcibly assume the second shape when heated above the transition temperature and then gently return, if not otherwise restricted, to the first shape when cooled to below the transition temperature.
Training an SMA object entails imparting shape memory into the object by restraining the object into a particular shape and thermally setting that shape, for example by heating the object to five hundred degrees Celsius or more. Two-way training typically entails thermo-mechanical cycles wherein the object is forced into the desired martensitic and austenitic shapes at respective low and high temperatures. The most commonly used SMA appears to be an alloy called Nitinol that is approximately fifty six percent (by weight) nickel and forty four percent titanium. Other available shape memory alloys include copper-zinc-aluminum, and copper-aluminum-nickel. The transition temperature of an SMA is highly sensitive to the composition of the alloy and can be selected by slightly varying the constituent ratios. Transition temperatures for Nitinol can reportedly be selected between negative one hundred and fifty degrees Celsius and positive one hundred and fifty degrees Celsius. Transformations typically take place over several minutes of time.
Applying these principles, SMA materials are utilized in constructing devices that do work when heated to transform from martensitic to austenitic phases. SMA materials are typically slower acting than such devices as electromechanical motors and hydraulic and pneumatic actuators, but SMA materials are highly reliable, generally require relatively few parts, and are weight efficient when compared to motors and the like. An object such as a wire formed of an SMA can be trained, for example, to shorten when heated. Such a wire can pull a load to do work. The wire can be heated by ohmic heating produced by passing an electrical current through the wire. Thus a current-actuated tensional actuator can be constructed of less parts than a conventional winching system. Typical available SMA devices include flexible wires or fibers that shorten when heated. While an object formed of an SMA can as well be trained to expand when heated, training rigid SMA members to expand longitudinally to do work when heated does not appear to be an avenue well explored by currently available SMA devices which include rotary and linear devices.
Rotary SMA devices are described in the U.S. Pat. No. 6,065,934, issued to Jacot et al. on May 23, 2004, and in the U.S. Pat. No. 6,499,952B1 issued to Jacot et al. on Dec. 31, 2002, which patents are incorporated herein by reference. The described rotary devices include torque tubes, formed with SMA materials, that twist when heated. The torque tubes work in conjunction with return springs that rotationally reset the tubes when thermal conditions allow. The described devices are useful in rotor-blade twist systems of helicopters and perhaps wherever thermally invoked torque would be useful.
Rotary and linear SMA devices are described in the U.S. Patent Application Publication No. US2002/0185932A1 of Gummin et al., published on Dec. 12, 2002, which patent application publication is incorporated herein by reference. A linear device described includes an arrangement of parallel bars interconnected by SMA wires that shorten when heated. The device effectively sums the decrements in the lengths of the wires by moving a bar a greater distance than any one wire shortens. The reference refers to this benefit as stroke amplification. The bars, however, do not exhibit SMA properties and so the stroke amplification gained, and the work accomplished by the device, are derived from the properties of the SMA wires.
A need exists for an SMA linear actuator that provides improved stroke amplification by deriving gains from interconnected members of an assembly. A need exists for linear actuator assemblies that are adaptable to both pushing and pulling applications.